This invention relates generally to gas turbine engines, and more particularly to apparatus and methods for sealing shrouds made of a low-ductility material in the turbine sections of such engines.
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine (also referred to as a gas generator turbine) includes one or more rotors which extract energy from the primary gas flow. Each rotor comprises an annular array of blades or buckets carried by a rotating disk. The flowpath through the rotor is defined in part by a shroud, which is a stationary structure which circumscribes the tips of the blades or buckets. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and is should generally be minimized.
It has been proposed to replace metallic shroud structures with materials having better high-temperature capabilities, such as ceramic matrix composites (CMCs). These materials have unique mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMCs have a coefficient of thermal expansion (“CTE”) in the range of about 1.5-5 microinch/inch/degree F., significantly different from commercial metal alloys used as supports for metallic shrouds. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/degree F.
CMC shrouds may be segmented to lower stresses from thermal growth and allow the engine's clearance control system to work effectively. The segment end faces or “slash” faces can have end gaps at operating conditions, with CMC shrouds potentially having larger gaps than comparable metal shrouds. Because of the slash face gaps, seals must be used to limit the leakage flow and protect the back flow margin of the shroud.
One type of segmented CMC shroud incorporates a “box” design eliminating the conventional shroud hangers which are used to mount prior art metallic turbine shrouds. In prior art practice, the shroud hanger is usually used to meter cooling air flow supplied to the shroud by removing a significant portion of the cooling air pressure. Because a box shroud will not have a shroud hanger, the outboard portion of the shroud could interface with very high air pressures.